One-sided welding is practiced extensively in such industries as the shipbuilding and boilemaking industries where large plates have to be welded together along abutting edges. Such welding technique is also often used in the manufacture of metal pipes where a metal strip is suitably formed and then welded along abutting edges.
Generally the technique involves aligning the edges of the workpiece or pieces in abutting relationship. A backing may then be located in position on one side of the work piece or pieces to overlie the weld line defined by the joint and then firmly pressed against the abutting edge zones of the work piece or pieces. Such backing may either be of the copper type, the copper flux type, the refractory backed flux type or the powder flux type. In other cases the backing may be located in position on a suitable work bench and the abutting edge zones may be laid on the work bench so that the backing underlies the weld line formed between them.
Alternatively, a suitably shaped refractory backing is used, for instance in the manufacture of pipes, where it is inserted into the pipe confines to underlie the weld line.
The backing may either have a groove extending along its length which is brought into register with the gap between the abutting edges or else the backing may be manufactured to allow the formation of a groove opposite the weld line during the welding operation.
Arc, beam or the like welding apparatus is then passed down along the gap to form a metal bath in the gap. By virtue of the nature of the backing a "reverse" bead is formed on the backing.
In a variation of this technique, the plates are held in overlapping relationship. The backing is then positioned against one of the plates and the arc, beam or the like welding apparatus is moved intermittently down the other plate, along a weld line in register with the backing, to melt the sheets in localized zones along the weld line and thereby bring about welding together of the sheets, as in spot or tack welding, with the difference that a controlled reverse bead is formed.
Such techniques suffer from a number of disadvantages.
In the first place, the backing remains in position over the reverse bead until the entire welding operation has been completed. As a result, the weld can only be inspected once welding has been completed. If, for any reason, there is a breakdown in the apparatus or the apparatus malfunctions, the defect is only discovered once the welding operation has been completed.
Furthermore, the techniques outlined above involve the use of a substantially rigid backing. This has limited the applicaton of the aforementioned techniques to generally straight line welds and, more particularly, over a substantially flat work piece, unless the apparatus is designed for a particular work piece.
Generally, also, pressure is applied to the backing at points intermittent along the length of the backing. Particularly where there are irregularities present in the plate, for instance, differences in thickness or bent positions, leaking or "burn through" may result. Also, the nature of the apparatus employed to date has required accurate alignment of the ends to be welded together. In the case of big plates, such as are encountered in the shipbuilding industry, this has necessitated enormous work benches, all adding to the manufactured cost of the end product.
In order to overcome some of the disadvantages, it is known to provide sensing apparatus which controls, by means of the welding power supply current output, the formation of the reverse bead through measuring radiation from the molten metal formed in the weld itself (such sensing apparatus is disclosed, for instance in U.S. Pat. Nos. 3,299,250 (Vilkas et al.) and 3,335,254 (Vilkas et al.)). Nevertheless; such sensing apparatus suffers from a number of disadvantages.
In the first place, its use is generally limited to one-sided welding techniques involving use of tungsten inert gas where the welding force is of a low magnitude. Where, however, the welding methods used have welding forces of greater magnitudes, such as are involved in metal inert gas welding and submerged arc welding, backings are required which are able to bridle the blow through. Generally the sensing apparatus cannot operate effectively through such backings.
Furthermore, the techniques described in the abovementioned patent specifications are generally limited to applications where it is possible to track accurately machined joints only with a sensor or to align the sensing device relative to the weld line. These techniques are not possible in the case of rough joints such as are encountered in ship building, boiler making, pipe manufacture and the like industries where the sensing device may stray from the weld line and give rise to defective welds.
An object of the present invention is the provision of apparatus and a method in which these disadvantages are overcome to a large extent.